1. Field of the Invention
The present invention relates to a projector and an image forming optical system having an image forming element represented by a liquid crystal or DMD (Digital Micromirror Device: trademark of Texas Instrument Inc.)
2. Description of the Related Art
In the recent years, there has been appearing devices for enlarged projection by means of a liquid crystal projector or DMD (Digital Micromirror Device) projector, in place of CRT projector in a projector market. Among the projectors, in case of a rear projector projecting an image from back surface of the screen, there is a progressively increasing demand for reducing thickness and weight in constructional nature. On the other hand, even in case of a front type projector, associating spreading to domestic use, it is becoming necessary to permit enlarged projection even in a small room. There is increasing demand for wide view angle to obtain large screen even by enlarged projection at small distance.
As an approach for enlarged projection at small distance and widened view angle of the projector, the projector employing the reflection type image forming optical system has been disclosed in WO97/01787 (PCT/JP96/01767) and Japanese Unexamined Patent Publication No. 10-111458. The reflection type image forming optical system, such as those employed in the projector disclosed in the above-identified publications, generally has various advantages, such as (1) having no chromatic aberration, (2) having capability of reduction of thickness and size for capability of bending of an optical path, (3) capability of obtaining small internal reflection and high contrast, and (4) having capability of high resolution in simple construction. Therefore, various forms of the reflection type image forming optical systems have been proposed.
Among number of reflection type image forming optical systems, one example of the optical type image forming optical system which the inventors have proposed in the past, is illustrated in FIG. 10. FIG. 10 is a general illustration of a projector which is constructed with an illuminating optical system 1, a reflection type image forming element (or image pick-up element) 2 and a reflection type image forming optical system. The reflection type image forming element 2 is arranged on an image forming surface of the reflection type image forming optical system. The reflection type image forming optical element in FIG. 10 is constructed with a first reflection mirror 3a having a concave surface directed to an image forming surface (reflection type image forming element 2) and having a free-curved surface shape, a second reflection mirror 3b having a convex surface directed to a flux of light from the first reflection mirror and having a free-curved surface shape, a third reflection mirror 3c having a convex surface directed to a flux of light from the second reflection mirror and having a free-curved surface shape, and a fourth reflection mirror 3d having a concave surface directed to a flux of light from the third reflection mirror and having a rotation symmetric aspheric shape, arranged in sequential order from the side of the reflection type image forming element 2.
When the reflection type image forming optical system of FIG. 10 is employed in the projector or the enlarged projection type display apparatus, since the shapes of the reflection surfaces of the reflection mirrors forming the reflection type image forming optical system are a free-curved surface shape, there is severe condition in precision in manufacturing and precision in assembling to the enlarged projection type display apparatus.
The present invention has been worked out in view of the shortcoming in the prior art set forth above. Therefore, it is an object of the present invention to provide a reflection type image forming optical system and a projector which is compact with wide view angle, in expensive and having uniform illumination intensity.
According to the first aspect of the present invention, a reflection type image forming optical system comprises:
a first reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed to an image forming surface arranged an image forming element thereon;
a second reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the first reflection mirror;
a third reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed toward a flux of light from the second reflection mirror; and
a fourth reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the third reflection mirror;
the first, second, third and fourth reflection mirrors forming a telecentric optical system.
According to the second aspect of the present invention, a reflection type image forming optical system comprises:
a first reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed to an image forming surface arranged an image forming element thereon;
a second reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the first reflection mirror;
a third reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the second reflection mirror; and
a fourth reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the third reflection mirror;
the first, second, third and fourth reflection mirrors forming a telecentric optical system.
In the preferred construction, the shapes of the reflection surfaces of the reflection mirrors forming the reflection type image forming optical system satisfies the following formulae (1) to (3) taking an optical axis as z axis, a plane perpendicular to the z axis as x-y plane, an intersection between the z axis and the x-y plane as origin O, and axes intersecting at the origin O, extending on the x-y plane and perpendicularly intersecting with each other as x axis and y axis:                     z        =                                            c              ⁢                              xe2x80x83                            ⁢                              ρ                2                                                    1              +                                                1                  -                                                            (                                              1                        +                        k                                            )                                        ⁢                                          c                      2                                        ⁢                                          ρ                      2                                                                                                    +                                    ∑                              i                =                1                            8                        ⁢                          xe2x80x83                        ⁢                                          α                i                            ⁢                              ρ                                  2                  ⁢                  i                                                                                        (        1        )                                          ρ          2                =                              x            2                    +                      y            2                                              (        2        )                                c        =                  1          /          r                                    (        3        )            
wherein xcex1i (i=1, 2, . . . , 8) are correction coefficients, r is a curvature radius of the reflection surface and k is conical coefficient.
According to the third aspect of the present invention, a projector comprises:
a telecentric illumination optical system having a light source and an image forming optical system;
a reflection type image forming element arranged on an image forming surface of the telecentric illumination optical system; and
a telecentric reflection type image forming optical system locating an image forming surface at an arrangement position of the reflection type image forming element and reflecting and projecting a flux of light reflected by the reflection type image forming element on a screen,
the reflection type image forming optical system including;
a first reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed to an image forming surface arranged an image forming element thereon;
a second reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the first reflection mirror;
a third reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed toward a flux of light from the second reflection mirror; and
a fourth reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the third reflection mirror.
According to the fourth aspect of the present invention, a projector comprises:
a telecentric illumination optical system having a light source and an image forming optical system;
a reflection type image forming element arranged on an image forming surface of the telecentric illumination optical system; and
a telecentric reflection type image forming optical system locating an image forming surface at an arrangement position of the reflection type image forming element and reflecting and projecting a flux of light reflected by the reflection type image forming element on a screen,
the reflection type image forming optical system including;
a first reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed to-an image forming surface arranged an image forming element thereon;
a second reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the first reflection mirror;
a third reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the second reflection mirror; and
a fourth reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the third reflection mirror.
According to the fifth aspect of the present invention, a projector comprises:
a telecentric illumination optical system having a light source and an image forming optical system;
a transmission type image forming element arranged on an image forming surface of the telecentric illumination optical system; and
a telecentric reflection type image forming optical system locating an image forming surface at an arrangement position of the transmission type image forming element and reflecting and projecting a flux of light passed through the transmission type image forming element on a screen,
the reflection type image forming optical system including;
a first reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed to an image forming surface arranged an image forming element thereon;
a second reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the first reflection mirror;
a third reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed toward a flux of light from the second reflection mirror; and
a fourth reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the third reflection mirror.
According to the sixth aspect of the present invention, a projector comprises:
a telecentric illumination optical system having a light source and an image forming optical system;
a transmission type image forming element arranged on an image forming surface of the telecentric illumination optical system; and
a telecentric reflection type image forming optical system locating an image forming surface at an arrangement position of the transmission type image forming element and reflecting and projecting a flux of light passed through the transmission type image forming element on a screen,
the reflection type image forming optical system including;
a first reflection mirror having a rotation symmetric aspheric shape concave reflection surface directed to an image forming surface arranged an image forming element thereon;
a second reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the first reflection mirror;
a third reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the second reflection mirror; and
a fourth reflection mirror having a rotation symmetric aspheric shape convex reflection surface directed toward a flux of light from the third reflection mirror.
The shape of the reflection surface of the reflection mirror forming the reflection type image forming optical system of the projector is rotation symmetric aspheric shape expressed by the foregoing formulae (1) to (3).
In the projector set forth above, the depth of the projector can be reduced by providing a light path converting element represented by a reflection mirror, prism and so forth in the illumination optical system in the projector.